The extracellular
matrix and cell-
cell interaction
Principles of Genetics and Molecular Biology
Dr. Diala Abu-Hassan, DDS, PhD
School of Medicine
The extracellular matrix (ECM)
The extracellular matrix fills the spaces between cells and binds cells and
tissues together.
Composed of different types of proteins and polysaccharides
Types:
1. Basal laminae (basements membranes): thin, sheet-
like, structure upon which layers of epithelial cells rest
-It supports sheets of epithelial cells
-It surrounds muscle cells, adipose cells, and peripheral
nerves.
2. Connective tissues (more ECM)
- Loose network of proteins and carbohydrates
underneath epithelial cell layers where fibroblasts are
distributed.
- Others: bone, tendon, and cartilage.
Matrix structural proteins
Components:
1. Tough fibrous proteins embedded in a gel-like polysaccharide ground substance.
2. Adhesion proteins that link components of the matrix both to one another and to attached cells.
Differences in the type and amount of different components make different types of ECM
Cartilage contains a
high concentration of
polysaccharides that
form a firm
compression-resistant
gel.
Different ECM composition
In bone, the
extracellular matrix is
hardened by
deposition of calcium
phosphate crystals.
Tendons contain a
high proportion of
fibrous proteins.
Collagens: the major structural
protein of the ECM
The most abundant proteins in mammals (25% of the total
protein mass).
Long, stiff, triple-stranded helical structure made of three
α chains
A basic unit of mature collagen is called tropocollagen.
Rich in glycine (33%), proline (13%), and hydroxyproline
(9%)
Sequence is (Gly-Pro- hydroxypro)
Hydroxyl groups stabilize the structure by hydrogen bond
formation
Contains hydroxylysine (attachment of polysaccharides)
Crosslinking of chains via lysine and hydroxylysine
Types of collagens
30 collagen genes that form >20 different types of collagen that resist tissue
stretching.
Types:
Fibril-forming (fibrillary) collagens such as collagen I
Fibril-associated collagens (link collagen fibrils to each other and to ECM
components, e.g. collagen types IX and XII link fibrils to one another and
to other components in the ECM).
Network-forming collagens, flexible because they are interrupted by non
helical short domains e.g. type IV is a constituent of the basal laminae.
Anchoring fibrils associate network-forming collagens to fibrillar collagens
Transmembrane collagens participate in cell matrix interactions
Assembly of fibrillar collagens
After secretion, they
assemble into collagen fibrils.
Collagen fibrils
aggregate into
collagen fibers
Synthesis of collagen
in the ER
Procollagen formation
Exocytosis
By procollagen peptidases
tropocollagen ECM
Synthesis of collagen
Individual collagen polypeptide chains are synthesized into
the endoplasmic reticulum (ER) as pro- chains.
In the ER, selected prolines and lysines are hydroxylated and
some of the hydroxylysines are glycosylated.
One chain combines with two others forming procollagen.
During or following exocytosis, extracellular enzymes, the
procollagen peptidases, remove the N-terminal and C-terminal propeptides forming tropocollagen (or simply
collagen).
Excision of both propeptides allows the collagen molecules to
polymerize into normal fibrils in the extracellular space.
Collagen-related diseases
Scurvy
Deficiency of vitamin C results in
insufficient formation of hydroxyproline and, hence, poor synthesis of collagen,
formation of unstable triple
helices.
Non-hydroxylated procollagen chains are then degraded
within the cell.
Symptoms: skin and gum lesions
and weak blood vessels
Collagen-related diseases
Osteogenesis imperfect (OI)
(Brittle-bone disease)
“Osteogenesis imperfecta" = imperfect bone
formation
A genetic disorder that cause fragile, soft, brittle,
and easily broken bones due to mutations in
COL1A1 and COL1A2 genes that interfere with the
assembly of type I collagen.
Four types of OI designated as type I through type IV
Type I: the mildest form of the condition
Type II: the most severe form that results in death
in utero or shortly after birth
Milder forms generate a severe crippling disease
Autosomal dominant pattern of inheritance
One copy of the altered gene is sufficient to cause the condition
Collagen-related diseases
Chondrodysplasias
Mutations affecting type II collagen cause chondrodysplasias, characterized by abnormal cartilage, which leads to bone and
joint deformities.
Collagen-related diseases
Ehlers-Danlos syndrome
A heterogeneous group of disorders that affect the skin, bones,
blood vessels, and other organs.
The signs and symptoms vary from mild to life-threatening.
All result from defects in collagen synthesis and/or processing.
Mutations in type I, III, or V collagen or in the synthesis of collagen
processing enzymes like procollagen N-peptidase, or lysyl
hydroxylase
Major manifestations:
Skin fragility and
hyperextensibility
Joint hypermobility.
Collagen-related diseases
Type III EDS
The most clinically important mutations are found in the gene of
type III collagen.
Since type III collagen is a major component of arteries, mutations
affecting type III collagen result in fragile blood vessels.
Symptoms
Stretchy skin Hypermobile joints
Elastic fibers structure
Abundant in organs to allow them to stretch
then return to the original shape, e.g. lungs
The main component of elastic fibers is elastin
Rich in proline and glycine.
Contains hydroxyproline, but no hydroxylysine
Not glycosylated
Elastin has two types of short segments that alternate along the
polypeptide chain
1. Hydrophobic segments, which are responsible for the elastic properties of
the molecule
2. Alanine- and lysine-rich a-helical segments, which form cross-links
between adjacent molecules
www.boundless.com
Formation of elastic networks
Crosslinking via covalent bonds between lysines
Secretion of tropoelastin
Assembly into elastic fibers
Elastic fiber in the arterial walls
Elastin is the dominant ECM
protein in arteries.
The normal elasticity of an
artery restrains the
proliferation of smooth
muscle cells.
Abnormal or deficiency of
the elastin results in
excessive proliferation of
smooth muscle cells in the
arterial wall and narrowing of the arteries.
Microfibrils
An elastin core is covered with
a sheath of microfibrils
Microfilrils are composed of a number of distinct
glycoproteins, including the
large glycoprotein fibrillin, which binds to elastin and is
essential for the integrity of
elastic fibers.
Present in hair and sperm tail.
Marfan's syndrome
Due to mutated fibrillin
Rupture of aorta.
Signs and symptoms:
A tall, thin build
Long arms, legs, fingers, and toes
Flexible joints
Scoliosis, or curvature of the spine
A chest that sinks in or sticks out
Crowded teeth
Flat feet
Emphysema
(destructive lung disease)
Due to a dysfunctional
alpha-1 antitrypsin leading to
increased activity of elastase
in lungs
The lysine to-glutamate
mutation causes protein
misfolding, formation of an
aggregate and block of ER
export.
Cigarette smoking also inactivates alpha 1-antitrypsin by oxidizing essential
methionine residues, decreasing the enzyme activity by a factor of 2000.
Matrix polysaccharides
Glycosaminoglycans (GAGs): Polysaccharides of repeated
disaccharides in which fibrous proteins are embedded
One is either N-acetylglucosamine or N-acetylgalactosamine
The other is usually acidic (either glucuronic acid or iduronic acid)
highly negatively charged
Modified by sulfate groups, e.g. dermatan sulfate, chondroitin
sulfate, keratan sulfate, heparin sulfate.
Exist as proteoglycans
Some are cell surface proteins with either transmembrane
domains (syndecans) or GPI anchors (glycipans) interacting with
integrins.
Hyaluronan: is the only GAG with a single long polysaccharide
chain.
Aggregan and hyalronan
The major proteoglycan of cartilage
A large proteoglycan consisting of more than 100 chondroitin sulfate
chains joined to a core protein.
Multiple aggrecan molecules
bind to long chains of hyaluronan
that become trapped in the
collagen network, forming large
complexes in the ECM of
cartilage.
Perlecan
Proteoglycans bind to matrix proteins forming gel-like networks
Heparan sulfate proteoglycan of basal lamina + collagen + laminin
Matrix adhesion proteins
Fibronectin
They link matrix proteins with one
another and to the surfaces of cells.
They interact with collagen and proteoglycans and specify matrix
organization and are major binding
sites for cell surface receptors such as integrins
Fibronectin: the principal adhesion protein of connective tissues.
A dimeric glycoprotein that is crosslinked into fibrils by s-s bonds.
Binds to collagen and GAGs
Binds to cell surface proteins like integrins linking cells to the ECM
Matrix adhesion proteins
Laminin
Laminins are tightly associated with another adhesion protein, called
nidogen, which also binds to type IV collagen, all of which form
crosslinked networks in the basal lamina.
Found in basal
laminae
T-shaped
heterotrimers with
binding sites for cell
surface receptors
(e.g. integrins), and
ECM components,
e.g. type IV
collagen, and
perlecan.
Cell-matrix interactions
Role of integrins
Integrins are a family of transmembrane heterodimers (α
and β)
They bind to short sequences present in ECM proteins
including collagen, fibronectin, laminin and proteoglycans.
Functions of integrins:
1. The major cell
surface receptors
that attach cells to
ECM
2. They anchor the
cytoskeleton at focal adhesions
and
hemidesmosomes.
Assembly of focal adhesions
1. Activation of integrin and binding to ECM 2. Recruitment of additional integrins forming focal complex
3. Development of small integrin clusters called focal complexes
4. Development of focal adhesions by the recruitment of formin,
talin, vinculin and α-actinin
Inactive integrins
can not bind the
matrix.
Cell-cell adhesion is a selective process
Cell Adhesion Molecules
Family Ligands
recognized Stable cell junctions
Selectins Carbohydrates No
Integrins
Extracellular
matrix
Focal adhesions and
hemidesmosomes
Members of Ig
superfamily No
Ig superfamily
Integrins No
Homophilic
interactions No
Cadherins Homophilic
interactions
Adherens junctions and
desmosomes
Homophilic vs. heterophilic
interactions
Selectin-mediated interaction between
leukocytes and endothelial cells
Intercellular
adhesion
molecules
Cadherins adhesion molecules
Selective adhesion between
embryonic cells and formation of
stable junctions between cells in
tissues.
Classic cadherins
E-cadherin: epithelial cells
N-cadherin: neural cells
P-cadherin: placental cells
Several subfamilies
Classic, Desmosomal, fat-like and
7-Transmembrane cadherins
Adherens junctions
Classic
cadherins
Desmosomes
Desmosomal
cadherins
heterophilic interaction
Molecular composition of tight
junctions
A network of protein strands that
continues around the entire
circumference of the cell
Each strand in these networks is
composed of transmembrane
proteins (claudins, occludin, and
JAMs) that bind to similar proteins
on adjacent cells, thereby sealing
the space between their plasma
membranes
These proteins interact with the
actin cytoskeleton via zonula
occludens protein.
Separate the apical part from the basolateral part of membranes
Prevent the passage of molecules (including ions )between epithelial cells
Gap junctions
They provide direct connections
between the cytoplasms of adjacent cells as open channels allowing ions and small molecules
(<1000 Da) including signaling molecules to diffuse freely between
neighboring cells, but preventing
the passage of proteins and nucleic acids.
Present in cells like epithelial cells,
endothelia cells, cardiac cells and smooth muscle cells
Molecular composition of gap
junctions
They are made of transmembrane
proteins called connexins
Six connexins assemble to form a
connexon (a cylinder with an open aqueous pore in its center).
Two connexons on adjacent cells
make a gap junction
Gap junctions and electrical
synapses
Specialized connexins in
electrical synapse
Gap junction diseases
Mutations in different types of connexins result in many diseases such
as:
Charcot-Marie-Tooth disease (degeneration of peripheral
myelinated nerves)
Deafness: inability to rapidly exchange K+
Cataracts: inability to obtain nutrients from the lens epithelial cells
Skin disease